Britain appreciated Japan's assistance during World War I but felt Tokyo could have done more and sometimes questioned its loyalty, according to today's historians.
With the world marking 100 years since the start of World War I, Ian Nish, a professor emeritus of international history at the London School of Economics, said the relationship Japan shared with the other Allies was relatively normal, although there were concerns about its intentions in China.
He added that "Japan offered middling, not outright or full-throated support."